Soldering is a common method of joining objects together. Soldering is used to form both physical and electrical connection. It is common practice in solder joining to coat those objects to be joined with chemical flux. Solid solder is then placed between the flux coated objects. The solder is heated to a molten state. The molten solder coats those parts of the objects which have been coated with flux. Thereafter, the molten solder is cooled, forming a physical joint between the two objects. The purpose of the flux is to chemically reduce oxide which is on the surface of the objects to be joined and the oxide on the surface of the molten solder. The oxide which is on the surface of the objects to be joined will prevent molten solder from wetting those surfaces. The oxide on the surface of the molten solder also will prevent the molten solder from wetting the surface of the object to be joined. During the process of heating the solid solder to form a molten mass of solder, the chemical flux volatilizes and is burned off. The volatilization of the flux, however, causes certain undesirable effects. Flux which is not completely removed also has undesirable effects.
As the flux volatilizes the generated gas can become incorporated into the molten solder which, upon cooling, can have voids formed therein from trapped gas. These voids weaken the physical joint of the objects which are soldered together. Furthermore, if the solder joint is forming an electrical connection between the two electrical conductors, the voids decrease the cross-sectional area of the electrical connection, thereby increasing the contact resistance between the two conductors which have been joined.
Flux residue which is not completely volatilized can result in corrosion of solder joints and of the objects which have been joined by the solder joints. This corrosion can increase as a function of time, resulting in further weakening of the physical joint of the two objects joined by the solder. An electrical connection formed by solder joint corrosion due to flux residue can also increase contact resistance over time. Moreover, in the case of an electrical connection formed by the solder joint, voids can move from the interior of the solder joint to the interface of the solder and one of the objects joined because of electromigration effects. If a void is sufficiently large, an electrical opening can result in the solder joint.
In microelectronic application solder bonds are commonly used to form electrical connections, for example, between semiconductor chips and semiconductor chip packaging structures. In one typical application a semiconductor chip is mounted onto a packaging substrate in a flip-chip configuration. In a flip-chip configuration the surface of the semiconductor chip having contact pads thereon is placed facing the surface of a packaging substrate having contact pads thereon. The chip contact pads and the substrate contact pads are electrically connected by solder mounds, commonly referred to as C4s. Typically, the C4 is vacuum deposited onto the chip contact pads. In other typical applications, beam leads or wires are solder bonded to solder coated contact pads on a chip or a packaging substrate. Typically, the solder is vacuum deposited onto either the chip or packaging substrate contact pad.
The general trend in the microelectronics industry is to decrease the size of electrical interconnections to chips and to packaging substrates. Also, the space between contact pads on a chip or a packaging substrate is continually being decreased. Both the decrease in size and space between contact pads on a semiconductor chip or packaging substrate permit a larger number of electrical interconnections to be made between the packaging substrate and the semiconductor chip. Consequently, in a case of a chip mounted onto a packaging substrate in a flip-chip configuration, the C4 interconnections have smaller dimensions and are more closely spaced. Moreover, decreased C4 size results in a smaller space between the semiconductor chip and the packaging substrate. The combination of a smaller space between the chip and the packaging substrate and a smaller space between C4s provides room for the volatilized flux to escape from between the chips. This situation aggravates the flux residue removal problem for a semiconductor chip mounted in a flip-chip configuration on a packaging substrate. This situation is similar for solder bonding of beam leads or wires to closely spaced contact pads on a semiconductor chip or a packaging substrate. The closely spaced beam leads or wires decrease the space to permit the volatilized flux to escape, and for cleaning flux residue from the regions where the solder joints are to be formed.
As the solder joints in microelectronic applications decrease in size, the undesirable effects, such as voids and corrosion, become more serious. The cross-sectional area of the solder joints are smaller; therefore, the effect of voids on contact resistance increases. Moreover, as the solder joints become smaller they become more susceptible to the adverse effect of corrosion.
It is desirable to be able to form solder joints for microelectronic applications and other applications without the use of chemical flux.
Howard, Jr. U.S. Pat. No. 4,646,958 describes a fluxless soldering process using a silane atmosphere. Solder reflow or solder chip joining is accomplished in a fluxless system by conducting the solder reflow chip joining procedure in a carrying gas which comprises from about 0.1 to about 10 percent by volume of silane (SiH.sub.4). A preferred carrier gas for the reflow is hydrogen, and a preferred carrier gas for the chip joining is nitrogen. The components containing the solder which is to be reflowed, or the semiconductor chip on the substrate to which it is to be joined, are placed within a furnace which is isolated from the external environment. The furnace is then filled with the carrier gas containing silane. The solder is then reflowed or the chip joining process is performed. Howard, Jr. includes silane in the carrier gas to reduce oxides formed on the solder surface, e.g., stanic oxide or lead oxide, back to tin and lead, respectively.
Silane is a highly explosive and toxic gas. It is highly desirable for a large scale manufacturing process to avoid the use of silane. The method of Howard, Jr. requires that the components containing the solder which are to be solder joined be contained within a closed furnace to provide the special atmosphere. This, therefore, requires that the furnace be opened and closed to place the components in the furnace and to remove the components from the furnace.
It is an object of this invention to provide a method of fluxless solder joining without the use of silane.
It is another object of this invention to provide a method of fluxless solder bonding without requiring that the components being solder joined be inclosed in a special atmosphere.
One application of the present invention is in tape automated bonding (TAB) technology. In TAB technology, a spaced series of beam lead sets are fabricated on an elongated flexible carrier film which can be wound onto and unwound from reels for automated fabrication of electronic components. The beam leads at each beam lead set project inwardly towards a central aperture in the flexible film into which they extend in cantilevered fashion. The inner ends of the beam lead which extend into the aperture can be electrically connected to contact pads on a semiconductor chip. The inner ends of the the beam leads can be fluxlessly solder bonded to the chip contact pads according to the methods of this invention. The outer ends of the beam leads extend over additional apertures in the carrier film. The outer ends of the beam leads can be fluxlessly solder bonded to contact pads on a substrate, such as a printed circuit board, by the method according to the present invention. The use of TAB-type tape permits an automated process of forming the inner and outer bonds.
It is another object of this invention to provide an apparatus for fluxlessly forming solder bonds.
In the method and the apparatus according to the present invention, a gas is directed at a solid mound of solder held between the objects to be soldered. The gas is sufficiently heated to melt the solid solder and has a sufficient momentum to disperse the surface oxide formed on the molten solder. The bonding process can be achieved in an oxidizing atmosphere such as atmospheric air.
It is therefor another object of this invention to provide a method and apparatus for fluxlessly solder bonding in an oxidizing environment.
Because, according to this invention, the components being joined do not have to be inclosed within a special environment, an automated process of solder reflow and solder joining of components without the use of flux can be achieved.
The methods generally taught by the following references, all of which are directed to using a heated gas for solder bonding, have not been found to achieve fluxless solder bonding: U.S. Pat. Nos. 3,912,153, 4,564,135, 4,552,300, 4,295,596, 4,605,152, 4,426,511; 3,754,698, Japanese Patent No. 55-86037 and IBM TDB Vol. 11, No. 10,; Mar. 1969.